1. Field of the Invention
This invention relates to a method and apparatus for varying the stiffness of a suspension bushing in a motor vehicle. More particularly, the method and apparatus are useful for selecting a bushing stiffness based upon an operating state of a transmission.
2. Disclosure Information
Vehicle ride and handling performance are strongly influenced by the operative characteristics of the various bushings utilized within the automotive vehicle suspension system. Changes in the spring rate or stiffness value of a particular bushing can directly influence a variety of operating characteristics, such as vehicle understeer, oversteer and upward squat characteristics, as well as chassis noise, vibration, and vehicle ride harshness.
For instance, it has been observed that varying the stiffness of suspension bushings on a driven axle can significantly improve the ride and shift quality as perceived by an operator. Specifically, optimal ride quality demands a suspension bushing having a relatively low spring rate. This permits the suspension to absorb disturbances in the roadway, such as tar strips, chuck holes, etc., without transmitting jarring vibrations to the occupants of the vehicle. On the other hand, optimal operation of the vehicle during a transmission shift event demands a bushing having a relatively high stiffness.
In connection with the set-up and calibration of automotive vehicle suspension systems, the spring rates of the bushings are determined by means of a lengthy trial and error process. In some cases, compromises must be made when choosing the final stiffness of a single, conventional bushing which exhibits a fixed-stiffness. This compromise has been avoided by providing a non-linear or variable rate bushing. One example of such a bushing includes fluid-filled hydrobushings.
One type of conventional, non-linear or variable rate fluid-filled hydrobushing comprises the use of electrorheological (ER) fluids incorporated within the bushing or mount. Electrorheological (ER) fluids consist of electrically polarizable particles suspended in an insulating fluid. By applying a large electric field to the electrorheological fluid, electric dipoles and higher order multipoles are induced within the suspended particles. The dipolar or multipolar particles experience mutual attraction with respect to each other whereby the particles are caused to form chains or other aligned structures that enable the fluids to exhibit solid-like mechanical properties. Removal or termination of the electric field causes the field-induced solid to revert to its initial non-induced fluid state. These dramatic, controllable changes in the rheological or mechanical properties of such bushings enables the construction of electromechanical devices or components which can be incorporated within various automotive subsystems.
Another type of conventional non-linear or variable rate fluid-filled hydrobushing comprises the use of ferrofluids or magnetorheological (MR) fluids incorporated within the bushing. Ferrofluids or magnetorheological fluids are the magnetic analogs of electrorheological fluids, and are produced by suspending magnetizable particles within a carrier fluid of low magnetic permeability, In use, a magnetic field is used to induce the desired changes in the suspension's rheological or mechanical properties through means of polarization and interparticle interactive principles of physics which are similar or analogous to those previously set forth briefly in connection with the electrorheological fluids. Consequently, the development of such magnetorheological fluids has likewise led to the construction of various electromechanical devices.
The aforenoted development of hydrobushings employing either electrorheological or magnetorheological fluids has led to the development and construction of desirable non-linear or variable rate electromechanical devices which can be effectively incorporated within the various automotive vehicle sub-systems. A major problem, however, has been encountered in connection with the employment of such hydrobushings using either electrorheological or magnetorheological fluids is that the particles dispersed or suspended within the working electrorheological or magnetorheological fluid tend to settle out of suspension over a period of time due, apparently, to the density mismatch between the particles and the host fluid. In addition, in view of the fact that such hydrobushing devices use fluids, the devices or components must necessarily be provided with appropriate seals in order to prevent any leakage of the electrorheological or magnetorheological fluid from the hydromount or hydrobushing. One further limitation on the application of the aforenoted devices is their ability to merely provide a resistive force, that is, damping.
A solution to the aforenoted problems encountered in connection with working electrorheological or magnetorheological fluids has been to embed, for example, the electrically polarizable particles within a viscoelastic solid, such as, for example, a polymer gel, in lieu of suspending or dispersing the particles within an insulating fluid. Operative characteristics of such electrorheological elastomers have proven to be as desired, that is, for example, the composite materials have exhibited solid-like properties, such as, for example, a non-zero shear modulus, even in the absence of an electrical field, and in addition, when the materials have been subjected to an electrical field, the shear modulus increases. The stiffness of such composite materials has therefore been able to be controlled electrically whereby such composite materials can be fabricated into or employed within various electromechanical devices.
Nevertheless, incorporation, fabrication and development of such composite materials in electromechanical devices in order to render such devices viable within the automotive environment, has proven problematical. For example, in order for such electrorheological elastomers to be operative, a source of high voltage is required. Therefore, an immediate concern not only arises in connection with safety procedures surrounding the generation, application, and control of such high voltage within the automotive environment, but in addition, the practical logistics of actually providing and incorporating such a source of high voltage within an automotive vehicle. Accordingly, it would be desirable to overcome the various operational disadvantages associated with the use of electrorheological (ER) elastomers.
Magnetorheological (MR) elastomers have been developed wherein magnetizable particles are dispersed within a viscoelastic solid so as to produce a composite material whose mechanical properties can be modulated by means of an applied magnetic field. In view of the fact that such magnetorheological (MR) elastomers are operative under low voltage magnetic fields, as opposed to high voltage electrical fields utilized in connection with electrorheological (ER) elastomers, the need for a source of high voltage is effectively eliminated. In particular, therefore, the voltage requirements for the magnetorheological (MR) elastomers can be readily satisfied within the automotive environment by means of the automotive vehicle electrical system.
It would be desirable to provide a method and apparatus for varying the stiffness of a suspension bushing incorporating a magnetorheological (MR) elastomers in response to specific transmission operating conditions which may be readily implemented within an motor vehicle.